Photovoltaic cell and a method for manufacturing the same

ABSTRACT

A photovoltaic cell is disclosed comprising a silicon substrate having two opposite main surfaces, wherein a first main surface of the two main surfaces is covered with a passivation layer stack, comprising a POx- and Al-comprising-layer covering the first main surface, and a capping layer which covers the POx- and Al-comprising-layer. Also disclosed is a method for manufacturing a photovoltaic cell.

FIELD

The invention relates to a photovoltaic cell and to a method formanufacturing a photovoltaic cell.

BACKGROUND

Photovoltaic cells with passivation layers are known from e.g.WO2016022026A1.

SUMMARY

A limiting factor in the performance of silicon based photovoltaiccells, also called solar cells hereafter, is recombination of chargecarriers at the surface, due to the presence of surface states whichfacilitate this recombination. Such surface recombination is an issuefor solar cells, since charge carriers which recombine at the surfaceare lost and cannot contribute to the collected current, therebyreducing energy conversion efficiency. In order to reduce surfacerecombination, the surface must be passivated such that therecombination activity of the surface states is suppressed. This iscommonly achieved by deposition or growth of a thin film layer or stackof dielectric or other materials on the semiconductor surface, such thatthe density of surface states at the interface between the semiconductorand this layer or stack is reduced. Commonly, this layer or stack alsoinduces band bending at the semiconductor surface, such that the surfaceconcentration of one type of charge carrier (either electrons or holes)is reduced. This also reduces surface recombination becauserecombination requires the presence of both carrier types.

Currently, a large number of materials are known to be effective forsemiconductor surface passivation. These include silicon oxide, siliconnitride, amorphous silicon, aluminium oxide, aluminium nitride, hafniumoxide, gallium oxide, titanium oxide, and tantalum oxide. Sometimesthese materials are combined in stacks.

Most of these materials result in upward band-bending (accumulation ofholes) at the semiconductor surface, due to the fact that they containnegative charge states either in the bulk material or near thesemiconductor interface (see FIG. 1). This is advantageous forpassivation of p-type semiconductor surfaces, but disadvantageous forpassivation of n-type semiconductor surfaces. This is because on n-typesurfaces such upward band-bending results in the formation of aninversion layer at the surface which facilitates lateral transport ofminority carriers to areas of high recombination such as edges, whichmay lead to shunting. On heavily doped n-type surfaces upwardband-bending may even increase rather than reduce surface recombinationbecause the induced band-bending is insufficient to produce fullinversion of the surface charge, but only results in depletion or weakinversion of the semiconductor surface.

Known passivation materials which induce downward band-bending(accumulation of electrons) at the semiconductor surface, and aretherefore advantageous for passivation of n-type surfaces, includesilicon oxide, and silicon nitride, due to the fact that both containpositive charge states. The former however only contains a rather lowconcentration of charge states (typically <5×10¹¹ cm⁻²), such that thecontribution of this induced band-bending to reducing surfacerecombination is rather small. Silicon nitride can have a much largercharge concentration (>5×10¹² cm⁻² [Hezel 1989]), however, the chargeconcentration in passivating silicon nitride films (those which alsofeature a low density of interface states) is typically somewhatsmaller, ranging between about 5×10¹¹ and 2×10¹² cm⁻² [Hezel 1989,Schuurmans 1996, Aberle 1999, Dauwe 2002, Wan 2013].

This is significantly smaller than the charge concentrations observedfor the best-passivating negatively charged material, aluminium oxide,which typically possesses negative charge concentrations in the range of2-10×10¹² cm⁻². There is therefore a gap between the availability ofpassivation materials with a strong negative charge and those with astrong positive charge.

A second important functionality of thin film materials in semiconductordevice fabrication is as dopant sources for the formation of highlydoped surface regions in a semiconductor substrate. This has for examplebeen demonstrated using deposited phosphorus- and boron-doped siliconoxides, as well as for phosphosilicate and borosilicate glass grownduring standard diffusion processes on silicon. Diffusion of dopantspecies into the semiconductor substrate may be induced by heating thesample surface to high temperatures, either across the whole surface asin a firing furnace, or only locally using for example laser-inducedheating (FIG. 2). A particularly advantageous case occurs when such thinfilm dopant sources also function as surface passivation layers. This isthe case for aluminium oxide, which both provides effective passivationof p-type surfaces due to its large negative charge, and has been shownto function as a dopant source for aluminium doping of silicon via laserdoping. This situation is advantageous because it allows the formationof local diffused contact regions which are self-aligned with thesurrounding passivation layers, thereby simplifying processing.

Aluminium and phosphorus sit on either side of silicon in the periodictable. Aluminium is a p-type dopant, and phosphorus an n-type dopant insilicon. Aluminium oxide Al₂O₃) (films are known to possess a largeconcentration of negative fixed charge when deposited on silicon.Stoichiometric aluminium phosphate (AlPO₄) can be considered to beisoelectronic with silicon oxide (SiO₂), which features a smallconcentration of positive charge at its interface with silicon.

The present invention relates to a photovoltaic cell comprising asilicon substrate having two opposite main surfaces, wherein a firstmain surface of the two main surfaces is covered with a passivationlayer stack, comprising:

-   -   a PO_(x)- and Al-comprising-layer covering the first main        surface, and    -   a capping layer structure which covers the PO_(x)- and        Al-comprising-layer.

The PO_(x)- and Al-comprising-layer is a layer comprising phosphorus,aluminium and oxygen, more particularly a layer comprising a mixture ofphosporus, aluminium and oxide. Such a passivation layer stack can betailored to obtain an optimal balance between the effective excesscarrier lifetime τ_(eff) (s), the fixed charge density Q_(f) (cm⁻²) andthe interface state density D_(it) (eV⁻¹cm⁻²). Preferably, the effectiveexcess carrier lifetime kf is relatively long and the and the interfacestate density D_(it) is relatively low. The desired fixed charge densityQ_(f) depends on the type of doping, i.e. n- or p-silicon, of the mainsurface onto which the PO_(x)- and Al-comprising-layer is applied.

For example, the PO_(x)- and Al-comprising-layer may possess a largepositive fixed charge, and may provide an excellent passivation ofcrystalline silicon surfaces. The silicon at the first main surface ofthe silicon substrate may be doped by the phosphorus in the PO_(x)- andAl-comprising-layer. By this means local, heavily doped regions may beformed which facilitate the formation of electrical contacts to thesilicon substrate. The capping layer structure acts as a moisturebarrier and provide chemical stability to the PO_(x)- andAl-comprising-layer which, dependent on its content, may be instablerelative to the environment.

In some cases, the mixed PO_(x)- and Al-comprising-layer which isembodied as a mixed AlP_(x)O_(y)-film may be stable and no capping layerhas to be applied in that case. In view thereof, the invention alsorelates to a photovoltaic cell comprising a silicon substrate having twoopposite main surfaces, wherein a first main surface of the two mainsurfaces is covered with a passivation layer stack comprising a PO_(x)-and Al-comprising-layer covering the first main surface, wherein thePO_(x)- and Al-comprising-layer is a mixed AlP_(x)O_(y)-film. Thereby,the ratio between the atomic percentage of phosphorus andphosphorus+aluminium (P/(P+Al)) in the PO_(x)- and Al-comprising layermay be chosen such that PO_(x)- and Al-comprising layer is stablerelative to the outside air. So the ratio is the number of atoms ofphosphorus divided by the number of atoms of phosphorus plus aluminium.

The following embodiments are embodiments of both the PV-cell with acapping layer as well the PV-cell without a capping layer. When thePV-cell does not have a capping layer, it is required that the PO_(x)-and Al-comprising-layer is stable relative to the outside air by itself.

In a first embodiment, the PO_(x)- and Al-comprising-layer is a mixedAlP_(x)O_(y)-film. The ratio between the atomic percentage of phosphorusand phosphorus+aluminium (P/(P+Al)) in the PO_(x)- and Al-comprisinglayer is thereby tailored to obtain an optimal balance between theeffective excess carrier lifetime τ_(eff) (s), the fixed charge densityQ_(f) (cm⁻²) and the interface state density D_(it) (eV⁻¹cm⁻²).

As is clear from FIG. 9b , the atomic percentage of phosphorus andphosphorus+aluminium (P/(P+Al)) in the layer has an impact on the fixedcharge density (Q_(f)). By tailoring this ratio, the fixed chargedensity can be varied from negative to positive charge, in the examplefrom −12*10 ¹² cm⁻² to +14*10 ¹² cm⁻². The effective excess carrierlifetime τ_(eff) is preferably long and this can also be obtained bytailoring the ratio between the atomic percentage of phosphorus andphosphorus+aluminium (P/(P+Al)) as is clear from FIG. 9a . The effectiveexcess carrier lifetime τ_(eff) can even be improved by forming gasannealing and by applying a capping layer of Al₂O₃ and optionallysubsequent annealing. The interface state density (D_(it)) is preferablyrelatively low and this can also be achieved by tailoring the atomicpercentage of phosphorus and phosphorus+aluminium (P/(P+Al)) in thePO_(x)- and Al-comprising layer.

In an embodiment, the ratio between the atomic percentage of phosphorusand phosphorus+aluminium (P/(P+Al)) in the layer is in the range of 0.6to 1.0. In this range, the effective excess carrier lifetime τ_(eff) isrelatively large, the fixed charge density Q_(f) is positive and theinterface state density D_(it) varies and may be relatively small. Thiswill result in a PV-cell with a good efficiency.

In an embodiment, the ratio between the atomic percentage of phosphorusand phosphorus+aluminium (P/(P+Al)) in the layer is at least 0.5.

In an embodiment, the PO_(x)- and Al-comprising-layer additionallycomprises at least one of H, Si, C.

For example H, may be beneficial for reducing the interface statedensity with the silicon substrate.

Si and additional Al may diffuse in the PO_(x)- and Al-comprising layerin the contact region between the PO_(x)- and Al-comprising layer andthe silicon substrate on the one hand and in the contact region betweenthe PO_(x)- and Al-comprising layer and a capping layer structure thatcomprises Al₂O₃ on the other hand.

In an embodiment, a thin SiO₂ layer (formed either intentionally or as aby-product of the deposition process) may be present between the Sisubstrate and the PO_(x) film so as to minimize the concentration ofelectrical interface states (D_(it)).

In an embodiment, this thin SiO₂ layer may have a thickness of 0.5-2 nm.

In an embodiment, the capping layer structure may be an aluminium oxide(A1 ₂O₃) layer. However, note that other materials could possibly beused in this role, especially other oxides or nitrides (e.g. siliconnitride, titanium oxide, silicon oxide etc.). The capping layerstructure may also be embodied as a stack of layers, for example a stackof an Al₂O₃-layer and SiN_(x)-layer

In an embodiment, the PO_(x)- and Al-comprising-layer covering the firstmain surface may have a thickness of less than 10 nm.

In an embodiment, the capping layer structure on top comprises analuminium oxide (Al₂O₃) layer having a thickness in the range of 2-30nm, more preferably in the range of 2-5 nm.

In an embodiment, a second main surface of the two main surfaces of thesilicon substrate may be covered with an Al₂O₃ layer.

The Al₂O₃ layer provides an excellent passivation of crystalline siliconsurface and possesses a negative fixed charge.

In an embodiment the photovoltaic cell having an Al₂O₃ layer on thesecond main surface may comprise a SiO₂ layer between the second mainsurface and the aluminium oxide (Al₂O₃) layer covering the second mainsurface.

In an embodiment this SiO₂ layer may have a thickness in the range of0.5-2 nm.

In a further elaboration of the embodiment with the aluminium oxide(Al₂O₃) layer covering the second main surface, the Al₂O₃ layer may bepart of a stack which comprises subsequently from the second mainsurface to the top, the Al₂O₃ layer, optionally a SiO₂-layer on top ofthe Al₂O₃ layer, and a SiN_(x) capping layer on top of that.

The present invention also relates to a method for manufacturing aphotovoltaic cell, the method comprising:

providing a silicon substrate having two opposite main surfaces;

applying a PO_(x)- and Al-comprising-layer on one of the main surfacesof the silicon substrate;

applying a capping layer structure on top of the PO_(x)- andAl-comprising-layer to cover the PO_(x)X- and Al-comprising-layer.

The PO_(x)- and Al-comprising-layer passivates the silicon and may betailored to obtain an optimal balance between the effective excesscarrier lifetime τ_(eff) (s), the fixed charge density Q_(f) (cm⁻²) andthe interface state density D_(it) (eV⁻¹cm⁻²). Preferably, the effectiveexcess carrier lifetime τ_(eff) is relatively long and the and theinterface state density D_(it) is relatively low. The desired fixedcharge density Qrdepends on the type of doping, i.e. n- or p-silicon, ofthe main surface onto which the PO_(x)- and Al-comprising-layer isapplied.

As stated above in relation to the PV-cell, in some cases, the mixedPO_(x)- and Al-comprising-layer which is embodied as a mixedAlP_(x)O_(y)-film may be stable and no capping layer has to be appliedin that case. In view thereof, the invention also relates to a methodfor manufacturing a photovoltaic cell, the method comprising:

providing a silicon substrate having two opposite main surfaces;

applying a PO_(x)- and Al-comprising-layer on one of the main surfacesof the silicon substrate, wherein the PO_(x)- and Al-comprising-layer isa mixed AlP_(x)O_(y)-film. Thereby, the ratio between the atomicpercentage of phosphorus and phosphorus+aluminium (P/(P+Al)) in thePO_(x)- and Al-comprising layer may be chosen such that the PO_(x)- andAl-comprising layer is stable relative to the outside air.

The following embodiments of the method according to the invention bothrelate to embodiments of the method for manufacturing the PV-cell with acapping layer as well as to embodiments of the method for manufacturingthe PV-cell without a capping layer. When the PV-cell does not have acapping layer, it is required that the PO_(x)- and Al-comprising-layeris stable relative to the outside air by itself.

In an embodiment, the PO_(x)- and Al-comprising-layer is a mixedAlP_(x)O_(y)-film which is formed by:

by applying a number of PO_(x)-layers and a number of Al-containinglayers, wherein the ratio between the atomic percentage of phosphorusand phosphorus+aluminium (P/(P+Al)) in the PO_(x)- andAl-comprising-layer is tailored to obtain an optimal balance between theeffective excess carrier lifetime τ_(eff) (s), the fixed charge densityQ_(f) (cm⁻²) and the interface state density D_(it) (eV⁻¹cm⁻²), whereinthe tailoring is effected by applying a desired number of PO_(x)-layersand a desired number of Al-comprising layers for forming the mixedAlP_(x)O_(y)-film.

By applying a desired number of PO_(x)-layers and a desired number ofAl-comprising layers, e.g. Al₂O₃-layers, the ratio P/(P+Al) within thePO_(x)- and Al-comprising-layer can be exactly obtained as desired.Especially when the layers are formed by ALD, for example in a spatialatmospheric system as the one marketed under the brand name Levitrack,the PO_(x)-layers and the Al₂O₃-layers are monolayers and the ratioP/(P+Al) within the PO_(x)- and Al-comprising-layer can be definedexactly.

In an embodiment, the ratio between the atomic percentage of phosphorusand phosphorus+aluminium (P/(P+Al)) in the layer is in the range of 0.6to 1.0.

In an embodiment, the PO_(x)- and Al-comprising-layer may formed byapplying at least one PO_(x)-layer and by applying at least oneAl-containing layer and subsequently anneal the stack of the at leastone PO_(x)-layer and the at least one Al-containing layer so as to mixthe Al and P to obtain a mixed AlP_(x)O_(y) film.

In an embodiment, the PO_(x)-layers and Al-comprising layers arealternately applied so as to form a stack of layers. Such an alternatingstack of PO_(x)-layers and Al-containing layers leads to a very wellmixed AlP_(x)O_(y) film after annealing.

In an embodiment, the application of at least one PO_(x)-layer and atleast one Al-containing layer and a subsequent PO_(x)-layer and asubsequent Al-comprising layer an intermediate anneal step is performedso as to mix the Al and P in the previously applied at least onePO_(x)-layer and at least one Al-containing layer to obtain a mixedAlP_(x)O_(y) film on top of which the subsequent PO_(x)-layer and asubsequent Al-containing layer are applied.

When annealing is done several times during the formation of theAlP_(x)O_(y) film, a very homogeneous content of the mixed AlP_(x)O_(y)film will be obtained.

In an embodiment, first the PO_(x)-layers and Al-comprising-layers arealternately applied so as to form the stack of layers and subsequentlythe stack of layers is annealed so as to mix the Al and P to obtain amixed AlP_(x)O_(y) film. In this embodiment, the annealing is done afterall the layers of the PO_(x)- and Al-comprising-layer are applied. Itmay depend on the type of layer application process whether intermediateannealing is preferred or whether annealing after application of theentire stack of PO_(x)-layers and Al-comprising-layers is preferred. Ina spatial atmospheric system as the one marketed under the brand nameLevitrack, the intermediate annealing is feasible without almost anydelay in the formation of the PV-cell.

In an embodiment, the applying of the PO_(x)- and Al-comprising-layer onthe first main surface may be effected by means of pulsed, spatial orbatch atomic layer deposition (ALD) or pulsed, spatial or batch chemicalvapour deposition (CVD).

In a further elaboration of this embodiment, the pulsed, spatial orbatch chemical vapour deposition (CVD) may be pulsed, spatial or batchplasma enhanced chemical vapour deposition (PECVD).

In an embodiment, the pulsed, spatial or batch atomic layer deposition(ALD) or pulsed, spatial or batch chemical vapour deposition (CVD) forthe applying of the PO_(x)-and Al-comprising-layer on the first mainsurface may be done by alternately exposing the surface to trimethylphosphate (TMP) and an O-containing reactant, chosen from the groupcomprising O2 plasma, H₂O, O₃ and H₂O₂ and alternately exposing thesurface to trimethyl aluminium (TMA) and an O-containing reactant,chosen from the group comprising O₂ plasma, H₂O, O₃ and H₂O₂.

In an embodiment, the first main surface may be exposed to a purge gas,such as N₂, between each exposure to trimethyl phosphate and exposure tothe O-containing reactant. Thereby, the first main surface may also beexposed to a purge gas, such as N₂, between each exposure to trimethylaluminium and exposure to the O-containing reactant.In an embodiment,the pulsed, spatial or batch atomic layer deposition (ALD) or pulsed,spatial or batch chemical vapour deposition (CVD) may performed in anatmospheric ALD or CVD systems, such as the spatial atmospheric systemmarketed under the brand name Levitrack.

In an embodiment, the pulsed, spatial or batch atomic layer deposition(ALD) or pulsed, spatial or batch chemical vapour deposition (CVD) maybe performed in a low-pressure deposition system.

With a low-pressure deposition system, a system is meant in which thepressure is subatmospheric.

In an embodiment, the applying of the PO_(x)- and Al-comprising-layer onthe first main surface of the silicon substrate may effected at atemperature in the range of 20-250° C., more preferably 80-120° C.

In an embodiment, the applying of the capping layer structure maycomprise the applying of an Al₂O₃ layer.

In an embodiment, the applying of the Al₂O₃ layer may effected in thesame atmospheric spatial ALD system as the applying of the PO_(x)- andAl-comprising-layer and immediately follows the applying of the PO_(x)-and Al-comprising-layer without removing the substrate from theatmospheric spatial ALD system so that the PO_(x)- andAl-comprising-layer is not exposed to air or moisture before the cappinglayer structure of Al₂O₃ is applied onto the PO_(x)- andAl-comprising-layer.

In an embodiment, the applying of the PO_(x)- and Al-comprising-layerand the applying of the Al₂O₃ layer may be effected in different modulesof an ALD, CVD and/or PECVD system, in which air exposure of the PO_(x)-and Al-comprising-layer is avoided by transferring the substrates from amodule in which the PO_(x)- and Al-comprising-layer is applied to amodule in which the Al₂O₃ layer is applied in an inert ambient or invacuum.

In an embodiment, the applying of the Al₂O₃ capping layer is effected ata temperature in the range of 20-300° C., more preferably 150-250° C.and even more preferably 180-220° C.

The invention is further elucidated in the detailed description withreference to the following figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Summary of passivating coatings on silicon in terms of fixedcharge Q_(f) and density of interface traps D_(it), taken from Cuevas etal. [Cuevas 2015], where we have added our data for PO_(x)/Al₂O₃ (theexact value of D_(it) and Q_(f) will depend on the details of thedeposition process conditions).

FIG. 2. Schematic illustration of laser doping process from a dopantcontaining film. The laser pulse locally heats the film, creating localopenings in the film and causing dopants to diffuse into the silicon,forming a localised heavily doped region at the surface.

FIG. 3a Diagram of a silicon surface passivated by Al₂O₃, which is acurrently known structure for surface passivation. Negative fixedcharges in the film repel negatively charged electrons from theinterface, resulting in effective passivation especially of p-typesurfaces.

FIG. 3b Diagram of a silicon surface passivated by the proposedPO_(x)/Al₂O₃ stack. Positive fixed charges in the film repel positivelycharged holes from the interface, resulting in effective passivationespecially of n-type surfaces.

FIG. 4a PO_(x) film thickness vs number of deposition process cycles ata temperature of 25° C. and 500 ms TMP dose time (bubbler temperature25° C.).

FIG. 4b PO_(x) film thickness vs number of cycles at 25° C. for dosetimes of 500 ms, 1 s, and 2 s (bubbler temperature 25° C.).

FIG. 4c Steady-state GPC vs TMP dose time at 25° C. (bubbler temperature25° C.).

FIG. 4d PO_(x) film thickness vs number of cycles at depositiontemperatures of 25, 100, and 200° C. and for bubbler temperatures of 25°C. and 70° C.

FIG. 5 Relative atomic composition determined by XPS vs sputter time fora PO_(x)/Al₂O₃ stack deposited at 100° C. on a polished (100) Sisubstrate.

FIG. 6a Effective excess carrier lifetime τ_(eff) (Δn=10¹⁵cm−3) vsannealing temperature for n-type Si (100) passivated by PO_(x)/Al₂O₃stacks deposited at 25° C. and by the same Al₂O₃ films without PO_(x).Annealing was performed consecutively for 10 minutes at each temperaturein N₂.

FIG. 6b Effective excess carrier lifetime τ_(eff) (Δn=10¹⁵cm−3) vsannealing temperature for n-type Si (100) passivated by PO_(x)/Al₂O₃stacks deposited at 100° C. and by the same Al₂O₃ films without PO_(x).Annealing was performed consecutively for 10 minutes at each temperaturein N2.

FIG. 7a Effective excess carrier lifetime τ_(eff) vs excess carrierconcentration Δn as a function of cumulative annealing temperature forn-type FZ Si (100) wafers passivated by PO_(x)/Al₂O₃ stacks deposited at25° C. Dashed lines show the intrinsic Si lifetime τ_(int) asparameterised by Richter et al [Richter 2012].

FIG. 7b Effective excess carrier lifetime τ_(eff) vs excess carrierconcentration Δn as a function of cumulative annealing temperature forn-type FZ Si (100) wafers passivated by PO_(x)/Al₂O₃ stacks deposited at100° C. Dashed lines show the intrinsic Si lifetime τ_(int) asparameterised by Richter et al [Richter 2012].

FIG. 8a High-frequency capacitance-voltage curves measured forPO_(x)/Al₂O₃ stacks deposited at 100° C. and the same Al₂O₃ filmswithout PO_(x) on silicon, where the samples were annealed at varioustemperatures for 10 min in N2 prior to metallisation.

FIG. 8b Corresponding fixed charge density determined from the data ofFIG. 8a as a function of annealing temperature.

FIG. 9a Effective excess carrier lifetime τ_(eff) vs the ratio betweenthe atomic percentage of phosphorus and phosphorus+aluminium (P/(P+Al))in the layer.

FIG. 9b Fixed charge density Q_(f) vs the ratio between the atomicpercentage of phosphorus and phosphorus+aluminium (P/(P+Al)) in thelayer.

FIG. 9c Interface state density D_(it) vs the ratio between the atomicpercentage of phosphorus and (phosphorus+aluminium) (P/(P+Al)) in thelayer.

DETAILED DESCRIPTION

PO_(x) films were deposited on a main surface of a silicon substrate inan atomic layer deposition (ALD) reactor at temperatures between 25 and100° C. by exposing samples alternately to trimethyl phosphate (TMP) andan O₂ plasma reactant in a cyclic fashion, with separating N₂ purges.This process resulted in linear thin film growth following an initialnucleation delay, with a growth-per-cycle (GPC) of between 0.5 and 1.1 Ådepending on the temperature and TMP exposure time (saturation of theGPC with respect to the precursor dose was not observed, at least at adeposition temperature of 25° C., so that this process should beconsidered a pulsed chemical vapour deposition process rather than trueALD). Note that other phosphorus-containing precursors, and otherreactants (e.g. H₂O, O₃, H₂O₂), could potentially be used in place ofTMP and O₂ plasma to deposit the PO_(x) layer in a similar manner. Itmay also be possible to use other deposition methods to deposit thePO_(x) layer in place of ALD, for example chemical vapour deposition,evaporation, sputtering, or solution-based methods.

Al₂O₃ capping layers were deposited in-situ immediately following PO_(x)deposition at the same temperature using trimethyl aluminium (TMA) andO₂ plasma. The Al₂O₃ films could in principle also be deposited usingother precursors or reactants, or by other deposition methods. The Al₂O₃capping layer could also possibly be replaced by another material withsuitable moisture barrier properties, for example silicon nitride ortitanium oxide.

Deposition of the passivation stacks was performed in a low-pressure(base pressure of ˜10 ⁻⁶ Torr) deposition system with a lowconcentration of water vapour. Alternatively, the stacks could also bedeposited in atmospheric ALD or CVD systems, such as in e.g. theLevitrack tool that is currently marketed for Al₂O₃ deposition. UncappedPO_(x) films were observed to visibly degrade within minutes on exposureto atmosphere, with the formation of fractal cracking patterns andblisters, presumably due to atmospheric moisture. In contrast, PO_(x)films capped by Al₂O₃ appeared to be stable over weeks and months ofatmospheric exposure. Gradual blister formation was observed for thickerPO_(x) films (˜5-10 nm) capped by Al₂O₃ in an as-deposited state.Annealing such stacks at a temperature of 200° C. in N₂ for 1 minuteimmediately following deposition resulted in improved stability andblister-free films. PO_(x) films thicker than 10 nm and capped by Al₂O₃exhibited significant cracking within minutes of exposure to atmosphereand significant blistering on annealing. Therefore, PO_(x) filmthickness is preferably kept below 10 nm.

To avoid air exposure of the rather reactive PO_(x) film, the depositionof the PO_(x)/Al₂O₃ stack could be carried out in one integrated processflow in a spatial ALD system such as the Levitrack tool. In such asystem different precursors can be injected at different segments alongthe track that are heated to different temperatures. In that way it ispossible to carry out the two processes at different temperature, e.g.PO_(x) at 100° C., and AlO_(x) at 200° C., without exposure to air inbetween.

X-ray photoelectron spectroscopy (XPS) compositional depth profiling(using sputtering) of PO_(x)/Al₂O₃ stacks (FIG. 5) showed the existenceof a PO_(x)- and Al-comprising-layer below the Al₂O₃ capping layer, i.e.AlP_(x)O_(y) layer. There is also a thin silicon oxide (SiO_(x)) layerlikely formed by O₂ plasma exposure during the initial deposition cycleswhen the PO_(x) is still nucleating. Carbon concentrations were belowthe detection limit (<1%) through the whole stack.

To investigate the passivation properties of these layers, 5-6 nm thickPO_(x) films were deposited in an atomic layer deposition (ALD) reactorfrom trimethyl phosphate (TMP) using an O₂ plasma reactant attemperatures of 25 and 100° C. 15 nm thick Al₂O₃ capping layers weredeposited in-situ at the same temperature using trimethyl aluminium(TMA) and O₂ plasma. Control samples featuring Al₂O₃ only (withoutPOO_(x)) were deposited in the same way. Symmetric carrier lifetime teststructures were fabricated on 280 μm thick double-side-polishedfloat-zone (100) 1-5 Ω cm n-type Si wafers, which received a standardRCA clean and HF dip immediately prior to PO_(x)/Al₂O₃ or Al₂O₃deposition. Following deposition, samples were annealed consecutively ata series of increasing temperatures for 10 minutes in N₂. Carrierlifetime measurements were performed using a Sinton WCT-120TSphotoconductance lifetime tester.

FIG. 6 shows the measured effective excess carrier lifetime τ_(eff) as afunction of post-deposition annealing temperature for samples passivatedby PO_(x)/Al₂O₃ or by Al₂O₃ only. Two points are immediately clear.Firstly, PO_(x)/Al₂O₃ is capable of providing excellent levels ofsurface passivation, comparable or better than that of plasma ALD Al₂O₃films deposited at the same temperature, with peak lifetimes of 1.9 msand 5.2 ms for stacks deposited at 25 and 100° C. respectively (itshould be noted that these temperatures are not optimal for Al₂O₃passivation). Secondly, the dependence of passivation quality onannealing temperature is fundamentally different for PO_(x)/Al₂O₃ stackscompared to Al₂O₃ alone. In particular, a significant increase inlifetime is observed at significantly lower annealing temperatures, withlifetimes on the order of 1 ms observed already after annealing for 10minutes at 250° C. The latter point shows clearly that the passivationprovided by the PO_(x)/Al₂O₃ stacks is not simply due to the Al₂O₃capping layer, and that the interfaces formed by these two materialswith c-Si are fundamentally dissimilar. The passivation of filmsdeposited at 25° C. was observed to degrade already on annealing at 400°C., but the lifetime of films deposited at 100° C. degraded only at 550°C.

Closer examination of the injection-dependent lifetime data (FIG. 7)reveals some further differences between films deposited at 25 and 100°C. The lifetime of samples passivated at 25° C. after annealing at 250°C. is well-described by a single-diode model, with a saturation currentdensity J₀ of 44 fA/cm² per side, suggesting that the surface isstrongly accumulated or inverted due to charge in the dielectric stack.Following annealing at higher temperatures the lifetime simultaneouslyincreases in high injection, while decreasing in low injection, suchthat the surface recombination can no longer be adequately parameterizedby J₀. In contrast, the stacks deposited at 100° C. exhibit single diodebehaviour over a much wider temperature range, with an exceptionally lowJ₀ of ˜6 fA/cm² per side observed after annealing at 450° C. Adeposition temperature of 100° C. therefore appears preferable. Thesedifferences in injection dependence may relate to differences in themagnitude of fixed charge. Interestingly, stacks deposited at bothtemperatures exhibit similarly high lifetimes in high injection, withcorresponding 1-Sun implied open-circuit voltages of ˜725 mV. Theserepresent exceptionally high values and show the outstanding passivationpotential of such stacks. The fixed charge concentration Q_(f) of thePO_(x)/Al₂O₃ stacks was determined using high-frequency (1 MHz)capacitancevoltage measurements (FIG. 8). A positive fixed charge ofbetween 4 and 4.6×10¹² cm⁻² was found for PO_(x)/Al₂O₃ stacks depositedat 100° C. and annealed at temperatures of 250-400° C., compared to anegative fixed charge of around 2×10¹² cm⁻² for the same Al₂O₃ filmswithout PO_(x). This charge is significantly larger than that ofstandard silicon nitride (SiN_(x)) films commonly used to passivaten-type silicon surfaces. The capacitancevoltage measurements alsoindicate that the investigated PO_(x)/Al₂O₃ stacks feature a very lowdensity of interface states (D_(it)), comparable or lower than that ofAl₂O₃, and significantly lower than that of SiN_(x), which along withthe large fixed charge accounts for the excellent passivation of thesestacks. This is likely due to the formation of a well-controlled SiOxinterface and effective hydrogenation by the capping Al₂O₃.

FIG. 9a shows the effective excess carrier lifetime σ_(eff) vs the ratiobetween the atomic percentage of phosphorus and phosphorus+aluminium(P/(P+Al)) in the layer. From this figure it is clear that the effectiveexcess carrier lifetime is high when the ratio P/(P+Al) is between0-0.15 and between 0.7-1.0. Forming gas annealing and the application ofa capping layer of Al₂O₃ improve the excess carrier lifetime so thateven in along the entire ratio range good values for the effectiveexcess carrier lifetime may be obtained.

FIG. 9b shows the fixed charge density Q_(f) (cm⁻²) vs the ratio betweenthe atomic percentage of phosphorus and phosphorus+aluminium (P/(P+Al))in the layer. From diagram it is clear that the fixed charge density canbe tailored from negative to positive as desired by changing the ratiobetween the atomic percentage of phosphorus and phosphorus+aluminium(P/(P+Al)) in the layer. In the range between 0.6-1.0 the fixed chargedensity is between 14 and 2. In this range of 0.6-1.0 a good value ofthe fixed charge density may be obtained and at the same time a very lowinterface state density may be obtained as well, which is advantageous.

FIG. 9c shows the interface state density D_(it) (eV⁻¹cm⁻²) vs the ratiobetween the atomic percentage of phosphorus and (phosphorus+aluminium)(P/(P+Al)) in the layer. A low interface state density and a relativelyhigh positive fixed charge density may be obtained when the ratiobetween the atomic percentage of phosphorus and phosphorus+aluminium(P/(P+Al)) in the layer is between 0.6-1.0.

The invention is not limited to the examples described in the detaileddescription.

LIST OF REFERENCES

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1. A photovoltaic cell comprising a silicon substrate having twoopposite main surfaces, wherein a first main surface of the two mainsurfaces is covered with a passivation layer stack, comprising: aPO_(x)- and Al-comprising-layer covering the first main surface, and acapping layer structure which covers the PO_(x)- andAl-comprising-layer.
 2. The photovoltaic cell according to claim 1,wherein the PO_(x)- and Al-comprising-layer is a mixedAlP_(x)O_(y)-film, wherein the ratio between the atomic percentage ofphosphorus and phosphorus+aluminium (P/(P+Al)) in the PO_(x)- andAl-comprising layer is tailored to obtain an optimal balance between theeffective excess carrier lifetime τ_(eff) (s), the fixed charge densityQ_(f) (cm⁻²) and the interface state density D_(it) (eV⁻¹cm⁻²).
 3. Thephotovoltaic cell according to claim 2, wherein the ratio between theatomic percentage of phosphorus and phosphorus+aluminium (P/(P+Al)) inthe layer is in the range of 0.6 to 1.0.
 4. The photovoltaic cellaccording to any one of claims 1-2, wherein the ratio between the atomicpercentage of phosphorus and phosphorus+aluminium (P/(P+Al)) in thelayer is at least 0.5.
 5. The photovoltaic cell according to any one ofclaims 1-4, wherein the PO_(x)- and Al-comprising-layer additionallycomprises at least one of H, Si, C.
 6. The photovoltaic cell accordingto any one of claims 1-5, comprising: a SiO₂ layer between the firstmain surface and the PO_(x)- and Al-comprising-layer.
 7. Thephotovoltaic cell of claim 6, wherein the SiO₂ layer is a layer having athickness in the range of 0.5-2 nm.
 8. The photovoltaic cell accordingto any one of claims 1-7, wherein the capping layer structure comprisesan aluminium oxide (Al₂O₃) layer.
 9. The photovoltaic cell according toany one of claims 1-8, wherein the capping layer structure comprises astack of layers, for example a stack of an Al₂O₃-layer and aSiN_(x)-layer.
 10. The photovoltaic cell according to any one of claims1-9, wherein is the capping layer structure comprises a layer chosenfrom the group consisting of SiN_(x), TiO_(x), SiO_(x).
 11. Thephotovoltaic cell according to any one of the preceding claims, whereinthe PO_(x)- and Al-comprising-layer covering the first main surface hasa thickness of less than 10 nm.
 12. The photovoltaic cell according toany one of the preceding claims, wherein the capping layer structure isan aluminium oxide (Al₂O₃) layer having a thickness in the range of 2-30nm, more preferably in the range of 2-5 nm.
 13. The photovoltaic cellaccording to any one of claims 1-12, wherein a second main surface ofthe two main surfaces is covered with an aluminium oxide (Al₂O₃) layer.14. The photovoltaic cell according to claim 13, comprising: a SiO₂layer between the second main surface and the aluminium oxide (Al₂O₃)layer covering the second main surface.
 15. The photovoltaic cell ofclaim 14, wherein the SiO₂ layer is a layer having a thickness in therange of 0.5-2 nm.
 16. The photovoltaic cell according to any one ofclaims 13-15, wherein the Al₂O₃ layer is part of a stack which comprisessubsequently from the second main surface to the top, the Al₂O₃ layer,optionally a SiO₂-layer on top of the Al₂O₃ layer, and a SiNx cappinglayer on top of that.
 17. A method for manufacturing a photovoltaiccell, the method comprising: providing a silicon substrate having twoopposite main surfaces; applying a PO_(x)- and Al-comprising-layer on afirst main surface of the two opposite main surfaces of the siliconsubstrate; applying a capping layer structure on top of the PO_(x)- andAl-comprising-layer to cover the PO_(x)- and Al-comprising-layer. 18.The method according to claim 17, wherein the PO_(x)- andAl-comprising-layer is a mixed AlP_(x)O_(y)-film which is formed by: byapplying a number of PO_(x)-layers and a number of Al-containing layers,wherein the ratio between the atomic percentage of phosphorus andphosphorus+aluminium (P/(P+Al)) in the PO_(x)- and Al-comprising-layeris tailored to obtain an optimal balance between the effective excesscarrier lifetime τ_(eff) (s), the fixed charge density Q_(f) (cm⁻²) andthe interface state density D_(it) (eV⁻¹cm⁻²), wherein the tailoring iseffected by applying a desired number of PO_(x)-layers and a desirednumber of Al-comprising layers for forming the mixed AlP_(x)O_(y)-film.19. The method according to claim 18, wherein the ratio between theatomic percentage of phosphorus and phosphorus+aluminium (P/(P+Al)) inthe layer is in the range of 0.6 to 1.0.
 20. The method according to anyone of claims 17-19, wherein the PO_(x)- and Al comprising-layer isformed by applying at least one PO_(x)-layer and by applying at leastone Al-containing layer and subsequently annealing the stack of the atleast one PO_(x)-layer and the at least one Al-containing layer so as tomix the Al and P to obtain a mixed AlP_(x)O_(y) film.
 21. The methodaccording to any one of claims 17-19, wherein the PO_(x)-layers andAl-containing layers are alternately applied so as to form a stack oflayers.
 22. The method according to claim 21, wherein between theapplication of at least one PO_(x)-layer and at least one Al-containinglayer and a subsequent PO_(x)-layer and a subsequent Al-containing layeran intermediate anneal step is performed so as to mix the Al and P inthe previously applied at least one PO_(x)-layer and at least oneAl-containing layer to obtain a mixed AlP_(x)O_(y) film on top of whichthe subsequent PO_(x)-layer and a subsequent Al-containing layer areapplied.
 23. The method according to claim 21, wherein first thePO_(x)-layers and Al-containing layers are alternately applied so as toform the stack of layers and subsequently the stack of layers isannealed so as to mix the Al and P to obtain a mixed AlP_(x)O_(y) film.24. The method according to any one of claims 17-23, wherein theapplying of the PO_(x)- and Al-comprising-layer on the first mainsurface is effected by means of pulsed, spatial or batch atomic layerdeposition (ALD) or pulsed, spatial or batch chemical vapour deposition(CVD).
 25. The method according to claim 18, wherein the pulsed, spatialor batch chemical vapour deposition (CVD) is pulsed, spatial or batchplasma enhanced chemical vapour deposition (PECVD).
 26. The methodaccording to claim 24 or 25, wherein the pulsed, spatial or batch atomiclayer deposition (ALD) or pulsed, spatial or batch chemical vapourdeposition (CVD) for the applying of the PO_(x)- and Al-comprising-layeron the first main surface is done by alternately exposing the surface totrimethyl phosphate (TMP) and an O-containing reactant, chosen from thegroup comprising O₂ plasma, H₂O, O₃ and H₂O₂ and alternately exposingthe surface to trimethyl aluminium (TMA) and an O-containing reactant,chosen from the group comprising O₂ plasma, H₂O, O₃ and H₂O₂.
 27. Themethod according to claim 26, wherein the first main surface is exposedto a purge gas, such as N₂, between each exposure to trimethyl phosphateand exposure to the O-containing reactant.
 28. The method according toclaim 26 or 27, wherein the first main surface is exposed to a purgegas, such as N₂, between each exposure to trimethyl aluminium andexposure to the O-containing reactant.
 29. The method according to anyone of claims 17-28, wherein the pulsed, spatial or batch atomic layerdeposition (ALD) or pulsed, spatial or batch chemical vapour deposition(CVD) is performed in an atmospheric ALD or CVD system.
 30. The methodaccording to any one of claims 27-28, wherein the pulsed, spatial orbatch atomic layer deposition (ALD) or pulsed, spatial or batch chemicalvapour deposition (CVD) is performed in a low-pressure depositionsystem.
 31. The method according to any one of claims 17-30, wherein theapplying of the PO_(x)- and Al-comprising-layer on the first mainsurface of the silicon substrate is effected at a temperature in therange of 20-250° C., more preferably 80-120° C.
 32. The method accordingto any one of claims 17-31, wherein the applying of the capping layerstructure comprises the applying of an Al₂O₃ layer.
 33. The methodaccording to claim 32, wherein the applying of the Al₂O₃ layer iseffected in the same atmospheric spatial ALD system as the applying ofthe PO_(x)- and Al-comprising-layer and immediately follows the applyingof the PO_(x)- and Al-comprising-layer without removing the substratefrom the atmospheric spatial ALD system so that the PO_(x)- andAl-comprising-layer is not exposed to air or moisture before the cappinglayer of Al₂O₃ is applied onto the PO_(x)- and Al-comprising-layer. 34.The method according to claim 32, wherein the applying of the PO_(x)-and Al-comprising-layer and the applying of the Al₂O₃ layer are effectedin different modules of an ALD, CVD and/or PECVD system, in which airexposure of the PO_(x)- and Al-comprising-layer is avoided bytransferring the substrates from a module in which the PO_(x)- andAl-comprising-layer is applied to a module in which the Al₂O₃ layer isapplied in an inert ambient or in vacuum.
 35. The method according toany one of claims 32-34, wherein the applying of the Al₂O₃ capping layeris effected at a temperature in the range of 20-300° C., more preferably150-250° C. and even more preferably 180-220° C.